In general, since air is supplied to a fuel cell system, the fuel cell system includes an air compressor and a contact bearing such as a ball bearing has typically been used therein. However, since the contact bearing has a limit of revolutions per minute due to friction of the bearing, an air foil bearing has been increasingly used to overcome the limitation of the revolutions per minute of the compressor. Meanwhile, when the air foil bearing is used, a high speed driving may be implemented since the limitation of the revolutions per minute due to the bearing is not present, a size of the compressor may be reduced, and noise may be reduced due to a non-contact driving of the air compressor.
However, when an amount of air required by the fuel cell system is minimal, an air supplying amount is reduced while driving revolutions per minute (rpm) of a turbo-type compressor using the air foil bearing is decreased. In particular, when the driving revolutions per minute (rpm) of the air compressor using the air foil bearing becomes a lift-off revolutions per minute region or less, contact occurs between a rotor and the bearing, which causes abrasion and damage of a bearing surface, thus deteriorating durability of the air compressor. In addition, since load capacity of the bearing for the rotor of the compressor is decreased, damage risk of parts due to collision between the rotor which is rotated and the bearing or a housing at the time of an external impact is increased.
Meanwhile, when the driving revolutions per minute (rpm) is increased to the lift-off revolutions per minute or greater even though the required amount of air is minimal, since the air supplying amount is increased, a dry out phenomenon may occur in which a humidification amount required for the fuel cell system is insufficient and a membrane electrolyte assembly (MEA), or the like becomes dry.